The present invention relates to a method of sealing a tube using a laser beam.
Certain types of articles, for example, discharge lamps, are hermetically sealed after charging the interior with various substances. As is well known, a discharge lamp has a discharge tube for radiating light, which is made of a transparent material such as quartz glass. The discharge tube for radiating light consists of electrodes arranged at the both ends thereof, an inert gas of a predetermined purity and a predetermined pressure necessary to start discharge, and a light-emitting material. The discharge lamp of this type is manufactured by a method comprising the steps of forming an exhaust aperture at part of the discharge tube having electrodes at both ends, evacuating the discharge tube through the exhaust aperture, introducing a predetermined inert gas and light-emitting material, and hermetically sealing the exhaust aperture.
As a method for hermetically sealing the exhaust aperture, a method which uses a laser beam is known as disclosed in, for example, Japanese Patent Publication (KOKOKU) No. 37913/75, British Patent Publication No. 1,298,397, and Japanese Patent Publication (KOKAI) No. 70582/78. This method comprises the steps of introducing a light-emitting material in a discharge tube having an exhaust aperture, setting the discharge tube inside a closed vessel, charging the closed vessel with a gas to be sealed in the discharge tube, and irradiating part of the discharge tube surrounding the exhaust aperture with a laser beam to melt this part and to seal the exhaust aperture.
This sealing method has many advantages, such as simple steps.
However, the sealing method using the conventional laser beam also has disadvantages to be described below, and improvements are desired. With the conventional method, the laser beam irradiates the opposing tube wall of the discharge tube through the exhaust aperture. Therefore, an impurity gas is introduced into the discharge tube. If the tube wall is excessively heated, it deforms. With a discharge lamp, especially with a high pressure discharge lamp, the starting voltage, the life of the discharge lamp, and other characteristics largely depend upon the purity of the gas inside the discharge tube. If the tube wall is deformed, the mechanical strength of the discharge tube may be reduced and the discharge characteristics may be degraded. If the shape of the discharge tube varies from one discharge tube to another, the mechanical strength and discharge characteristics also vary from one discharge tube to another. Especially with a discharge lamp of high efficiency and small size which has recently been developed, the variations in the shape of the discharge tube after sealing extremely adversely affect the discharge characteristics, the color temperature, the color rendering properties, and the luminous efficacy of a source. Therefore, a method for sealing a discharge tube is desired with which the discharge tube is not deformed.